magnetic drum

a digital computer memory in which the data carrier is a cylinder, coated with a layer of magnetic material, that rotates at a constant angular velocity. The cylinder is made of nonmagnetic alloys such as stainless steel; it is 100-500 mm in diameter and 300-700 mm long. The magnetic surface is made of Ni-Co or Co-W alloys, which are deposited by electrolysis. Data are recorded magnetically and read out by means of magnetic heads located along the generatrices of the cylinder at a distance of 15-30 microns (μ) from its surface.

Magnetic drums are classified as random-access devices. The data are distributed on “tracks,” which are portions of the sur-face spaced 0.2-0.8 mm apart; the recording density (25-40 impulses per millimeter) depends to a great extent on the gap between the heads and the surface of the magnetic drum. For gaps of several microns, careful balancing and alignment of the drum during mounting on its bearings, as well as insulation of the operating surface and heads from dust and moisture, are very important. The use of “floating” heads, which are not rigidly mounted but rather float on an air cushion at the surface of the magnetic drum, make possible reduction of the gap and an increase in recording density, as well as a reduction in the requirements for manufacturing tolerances and alignment of the drum.

The number of tracks on a magnetic drum ranges from tens to several thousand, the information capacity from 6 ×10⁵ to 8 ×10⁹ bits, the average access time (retrieval of information) from 2.5 to 50 millisec, and the rate of rotation from 500 to 20,000 rpm. In magnetic drums of small capacity the heads are fixed and usually equal in number to the number of digits in a machine word. To increase the recording density, the heads are mounted with an offset. In magnetic drums of high capacity, movable heads with an automatic shift are used, parts of a word (usually bytes), rather than an entire word, are recorded.